1. Field of the Invention
The present invention relates to a flat gasket made from a soft material, and in particular to cylinder head gaskets, exhaust flange seals or ancillary seals for internal-combustion engines, which are coated wholly or in part, on one or both sides, with a material which prevents their adhesion to the surfaces to be sealed in use.
2. Discussion of the Prior Art
A drawback of, in particular, soft material, flat gaskets for internal-combustion engines, is the adhesion of the gaskets to the sealed surfaces which develops during operation of the engine, because of the applied sealing pressure and the temperature stresses. When an internal-combustion engine requires repairs necessitating the removal of a gasket, the gasket is generally destroyed, because parts thereof continue to adhere to the flange surfaces which the gaskets were intended to seal. The adhering fragments must then be carefully removed from the flange surfaces and, generally, the old gasket cannot be reused.
It is therefore customary in the sealing art to avoid such damage to the gasket by coating the gaskets, wholly or in part, on one or both sides, with release coatings which, preferably, adhere to the gasket, but counteract adhesion of the gasket to the surface to be sealed. Silicone resins, polysiloxanes, polytetrafluoroethylene and other polymeric substances having a low coefficient of friction are disclosed as useful gasket coating materials in U.S. Pat. No. 4,223,897. However, coating materials having a low coefficient of friction generally do not adhere well to the gasket. Thus, according to the reference, it is necessary to have either a zone free of impregnating agent in the surface region of the soft material fiber mat of the impregnated gasket so that the coating material can anchor itself in the exposed fibers, or the surfaces of the impregnated gaskets to be coated must be slightly dissolved and softened, after the impregnating agent has been cross-linked, so that a good bond develops between the impregnated gasket and the coating material. The reference's teachings thus require relatively large amounts of these costly coating materials and additional process steps, however, so that gaskets produced in this manner are expensive; too expensive for mass produced articles.
It is also known in the sealing art to use graphite in a powdered to a fine-grained particulate forrm as a material to counteract sticking. For this purpose, graphite has been previously used in a suspension with aqueous latex. Too high a percentage of latex in the coating, however, renders the non-stick properties of the graphite ineffective, the coated gaskets stick to the sealed surfaces of the internal-combustion engine and the desired release effect is not realized. Although this drawback can be overcome by modifying the composition to limit the amount of latex to a smaller percentage, this tradeoff tends to result in the graphite no longer being sufficiently bonded to the gasket surfaces being coated, graphite is released and the gaskets give off color. This makes the coated gaskets difficult to handle and the loss of graphite corresponds to a diminishing of the sought-after non-stick properties, especially noticeable after longer periods of storage.